As nanoelectrospray ionization continues to grow and gain acceptance as a valuable research tool within the mass spectrometry field, there has been a push to produce nanoelectrospray emitters which are inexpensive and have long lifetimes. “Nanoelectrospray” was first developed by Wilm and Mann in 1994 (Wilm and Mann, “Electrospray and Taylor-Cone Theory, Dole's Beam of Macromolecules at Last?” Int. J. Mass Spectrom. Ion Processes, 136(1):167–180 (1994); Wilm and Mann, “Analytical Properties of the Nanoelectrospray Ion Source,” Anal. Chem., 68(1):1–8 (1996) ). Their technique used a pulled-glass substrate as the electrospray ionization emitter. Nanoelectrospray is a static technique and relies upon capillary action induced by the applied electric field to draw the solution to the emitter tip so that it can be electrosprayed (Wood et al., “Miniaturization of Electrospray Ionization Mass Spectrometry,” Applied Spectroscopy Reviews, 38(2):187–244 (2003) ); therefore, no forced flow (from a syringe pump or LC pump) is needed, and flow rates are generally in the tens of nanoliters per minute. The droplets produced have 100–1000 times less volume than those produced with conventional electrospray, and desolvation does not require the use of a nebulizing gas to aid the drying of the droplets. These advantages lead to increased sensitivity and decreased limits of detection, making nanoelectrospray the technique of choice when sample volumes and analyte concentrations are limited.
Initial nanoelectrospray emitters used pulled-glass substrates with a metal, typically gold, as the applied conductive medium (Wilm and Mann, “Electrospray and Taylor-Cone Theory, Dole's Beam of Macromolecules at Last?” Int. J. Mass Spectrom. Ion Processes, 136(1):167–180 (1994); Wilm and Mann, “Analytical Properties of the Nanoelectrospray Ion Source,” Anal. Chem., 68(1):1–8 (1996); Valaskovic et al., “Attomole-Sensitivity Electrospray Source for Large-Molecule Mass Spectrometry,” Anal. Chem., 67(20):3802–3805 (1995) ). However, these emitters suffered from the susceptibility of the conductive metal to be ablated from the glass surface through a coronal discharge (Wood et al., “Miniaturization of Electrospray Ionization Mass Spectrometry,” Applied Spectroscopy Reviews, 38(2): 187–244 (2003) ). Much work has been done to stabilize and protect these metal coatings in an effort to increase the emitter lifetime (Kriger et al., “Durable Gold-Coated Fused Silica Capillaries for Use in Electrospray Mass Spectrometry,” Anal. Chem., 67(2):385–389 (1995); Nilsson et al., “On-Column Conductive Coating for Thermolabile Columns Used in Capillary Zone Electrophoresis Sheathless Electrospray Ionisation Mass Spectrometry,” Rapid Commun. Mass Spectrom., 14(1):6–11 (2000); Valaskovic et al., “Long-Lived Metallized Tips for Nanoliter Electrospray Mass Spectrometry,” J. Am. Soc. Mass Spectrom., 7(12):1270–1272 (1996); Barnidge et al., “A Design for Low-Flow Sheathless Electrospray Emitters,” Anal. Chem., 71:4115–4118 (1999)). However, these techniques greatly add to the production time and cost of the emitter.
Another approach has been to use polymer-based systems, either a conductive polymer (Bigwarfe et al., “Polyaniline-Coated Nanoelectrospray Emitters: Performance Characteristics in the Negative Ion Mode,” Rapid Commun. Mass Spectrom., 16:2266–2272 (2002); Maziarz et al., “Polyaniline: A Conductive Polymer Coating for Durable Nanospray Emitters,” J. Am. Soc. Mass. Spectrom., 11(7):659–663 (2000); White and Wood, “A Unique Alternative Emitter for Low-Flow Electrospray Ionization,” Am. Biotechnol. Lab., 20:16, 18 (2002); White and Wood, “Reproducibility in Fabrication and Analytical Performance of Polyaniline-Coated Nanoelectrospray Emitters,” Anal. Chem., 75:3660–3665 (2003)) or a nonconductive polymer doped with conductive material (Nilsson et al., “A Simple and Robust Conductive Graphite Coating for Sheathless Electrospray Emitters Used in Capillary Electrophoresis/Mass Spectrometry,” Rapid Communications in Mass Spectrometry, 15(21):1997–2000 (2001); Wetterhall et al., “A Conductive Polymeric Material Used for Nanospray Needle and Low-Flow Sheathless Electrospray Ionization Applications,” Anal. Chem., 74:239–245 (2002)). These emitters have shown better resilience to electrical discharge and the longer lifetimes needed for the coupling of online separation techniques. These systems are generally cheaper than their metal counterparts, and some require less handling during manufacture.
Another approach has been to insert a metal wire into the untapered end of the emitter in order to make electrical contact with the solution (Fong and Chan, “A Novel Nonmetallized Tip for Electrospray Mass Spectrometry at Nanoliter Flow Rate,” J. Am. Soc. Mass Spectrom., 10(1):72–75 (1999); Van Berkel et al., “Electrochemical Processes in a Wire-in-a-Capillary Bulk-Loaded, Nano-Electrospray Emitter,” J. Am. Soc. Mass Spectrom., 12:853–862 (2001); Cao and Moini, “A Novel Sheathless Interface for Capillary Electrophoresis/Electrospray Ionization Mass Spectrometry Using an In-Capillary Electrode,” J. Am. Soc. Mass Spectrom., 8:561–564 (1997); Kelleher et al., “Unit Resolution Mass Spectra of 112 kDa Molecules with 3 Da Accuracy,” J. Am. Soc. Mass Spectrom., 8(4):380–383 (1997)). This eliminates the need for an external coating. This technique can become labor-intensive, and it might not be suitable for online separations like capillary LC. It has also been shown that the metal wire used as an electrode may undergo electrolysis and produce additional species whose ions further complicate the observed mass spectrum (Van Berkel et al., “Electrochemical Processes in a Wire-in-a-Capillary Bulk-Loaded, Nano-Electrospray Emitter,” J. Am. Soc. Mass Spectrom., 12:853–862 (2001)).
A third alternative has been to use carbon or graphite as the conductive medium. Many different types of carbon have been used, such as colloidal graphite (Zhu et al., “A Colloidal Graphite-Coated Emitter for Sheathless Capillary Electrophoresis/Nanoelectrospray Ionization Mass Spectrometry.” Anal. Chem., 74(20):5405–5409 (2002)), carbon particles glued to glass (Nilsson et al., “A Simple and Robust Conductive Graphite Coating for Sheathless Electrospray Emitters Used in Capillary Electrophoresis/Mass Spectrometry,” Rapid Communications in Mass Spectrometry, 15(21):1997–2000 (2001); Wetterhall et al., “A Conductive Polymeric Material Used for Nanospray Needle and Low-Flow Sheathless Electrospray Ionization Applications,” Anal. Chem., 74:239–245 (2002)), and even a soft pencil (Chang and Her, “Sheathless Capillary Electrophoresis/Electrospray Mass Spectrometry Using a Carbon-Coated Fused-Silica Capillary,” Anal. Chem., 72(3):626–630 (2000); Chang et al., “Sheathless Capillary Electrophoresis/Electrospray Mass Spectrometry Using a Carbon-Coated Tapered Fused-Silica Capillary with a Beveled Edge,” Anal. Chem., 73(21):5083–5087 (2001)). Early carbon emitters have shown the feasibility of using carbon as a conductive medium, but their manufacture suffers from many of the same drawbacks as other coating techniques.
The present invention is directed to overcoming these deficiencies in the art.